The present invention relates to a cylinder liner for insert casting used in a cylinder block, and an engine having the cylinder liner.
Cylinder blocks for engines with cylinder liners have been put to practical use. Cylinder liners are typically applied to cylinder blocks made of an aluminum alloy. As such a cylinder liner for insert casting, the one disclosed in Japanese Laid-Open Patent Publication No. 2003-120414 is known.
To meet the recent demand for lower fuel consumption, a configuration has been proposed in which distances between cylinder bores of an engine are reduced to lighten the engine.
However, reduced distance between the cylinder bores causes the following problems.
(1) Sections between the cylinder bores are thinner than the surrounding sections (sections spaced from the sections between the cylinder bores). Thus, when producing the cylinder block through the insert casting, the rate of solidification is higher in the sections between the cylinder bores than in the surrounding sections. The solidification rate of the sections between the cylinder bores is increased as the thickness of such sections is reduced.
Therefore, in the case where the distance between the cylinder bores is short, the solidification rate of the casting material is further increased. This increases the difference between the solidification rate of the casting material between the cylinder bores and that in the surrounding casting material. Accordingly, a force that pulls the casting material located between the cylinder bores toward the surrounding sections is increased. This is highly likely to create cracks between the cylinder bores (hot tear).
(2) In an engine in which the distance between the cylinder bores are short, heat is likely to be confined in the sections between the cylinder bores. Thus, as the cylinder wall temperature increases, the consumption of the engine oil is promoted.
Accordingly, the following conditions (A) and (B) need to be met when improving the fuel consumption rate through reduction of the distance between the cylinder bores.
(A) To suppress the movement of the casting material from the sections between the cylinder bores to the surrounding sections due to the difference in the solidification rates, sufficient bond strength needs to be ensured between the cylinder liners and the casting material when producing the cylinder block.
(B) To suppress the consumption of the engine oil, sufficient thermal conductivity needs to be ensured between the cylinder block and the cylinder liners.
According to the cylinder liner disclosed in Japanese Laid-Open Patent Publication No. 2003-120414, a film is formed on the cylinder, which film establishes metallic bond with the casting material of the cylinder block. This structure increases the bond strength between the cylinder block and the cylinder liner. However, it has been found out that, in the case where the cylinder block is produced using such a cylinder liner, relatively large gaps are formed between the cylinder block and the cylinder liner, resulting in a reduced thermal conductivity. This is though to be caused by insufficient bond strength between the cylinder liner and the casting material during the production of the cylinder block.